A compressor having a centrifugal downstream stage usually includes a rotary impeller. The impeller comprises a series of blades driven in rotation, and it is made in such a manner as to accelerate the gas passing therethrough.
The diffuser presents an annular space surrounding the impeller. The diffuser serves to reduce the speed of the gas leaving impeller, and as a result to increase its static pressure. Diffusers may be of the vane type or of the duct type.
In general, these two types of diffuser comprise a radially-oriented annular upstream portion presenting a series of diffusion passages connected to the outlet of the compressor in order to recover the accelerated gas leaving it. These diffusion passages are of a section that increases progressively from upstream to downstream in order to diffuse the flow of gas leaving the compressor. Diffusers of the vane type make use of a series of circularly spaced-apart vanes forming the diffusion passages between one another. In duct type diffusers, the passages are constituted by duct or pipe elements, e.g. formed between two joined-together opposite plates.
Downstream from the upstream portion, diffusers generally include an elbow-shaped annular intermediate portion for curving the flow path of the diffuser and bringing the flow of gas towards the combustion chamber.
Downstream from the intermediate portion, diffusers generally comprise an annular downstream portion made up of a series of circularly spaced-apart flow-straightening vanes for straightening the flow of gas, and thus for reducing or eliminating the circumferential swirling of the flow of gas as it leaves the diffusion passages, prior to said flow entering into the combustion chamber.
In general, the centers of the injection orifices of the combustion chamber are distributed around the axis of the turbomachine on a circle of radius R1, while the mean radius R2 of the downstream portion of the diffuser is greater than the radius R1.
In certain prior art turbomachines, the downstream portion of the diffuser follows the line of the outer casing of the chamber and is directed towards the outer zone that bypasses the chamber (i.e. the through zone between the chamber and the outer casing). In other words, in a section plane containing the axis of the turbomachine, the mean axis of the flow path at the outlet from the downstream portion of the diffuser is parallel to the mean axis of the bypass flow outside chamber. That solution is unsatisfactory since all of the main gas flow leaving the diffuser bypasses the combustion chamber on the outside prior to being shared between the outer flow and the flow that feeds both the chamber end wall and the inner bypass zone of the chamber (i.e. the through zone between the chamber and the inner casing). The injection systems and the inner bypass zone are then fed with a secondary flow diverted from the main flow, with such diversion giving rise to significant pressure drop (i.e. loss of pressure) between the outlet from the diffuser and the upstream end of the injection system, and between the outlet from the diffuser and the inner bypass zone.
The functional consequences of such pressure drop are the following:                when designing the turbomachine, the large pressure drop between the outlet from the diffuser and the injection system needs to be compensated by an overall increase in the pressure drop of the module between the outlet from the diffuser and the outlet of the chamber so as to conserve a pressure drop on passing through the injection system that is sufficient to ensure air-fuel mixing and combustion. This increase in the pressure drop of the module gives rise to an increase in fuel consumption.        the gas feed between the outer bypass zone and the inner bypass zone of the chamber is highly asymmetrical (the primary and dilution gas jets are more penetrating on the outside than on the inside), which makes it more difficult to control the temperature profile at the outlet from the chamber.        the poor gas feed to the inner bypass zone leads to a reduction in the gas flow speeds in devices for cooling the inner wall of the combustion chamber, thereby reducing convective heat exchange coefficients, and thus reducing the overall efficiency of said cooling.        poor gas feed to the inner bypass zone leads to a reduced overpressure ratio, decreasing the efficiency of the cooling of the turbine nozzle situated downstream from the chamber.        
In order to avoid those drawbacks, in other prior art turbomachines, such as the machine of document FR 2 372 965, the downstream portion of the diffuser is inclined relative to the axis of the turbomachine towards the combustion chamber in such a manner that, in a section plane containing the axis of the turbomachine, the mean axis of the flow path at the outlet from the downstream portion of the diffuser passes via the chamber end wall between the maximum radius and the minimum radius of the chamber end wall. The flow path is defined as being the envelope that defines the flow space for the gas, and thus the gas flow. In the downstream portion of the diffuser, the flow path is defined by the inner outline of said downstream portion.
Such an inclination of the downstream portion of the diffuser relative to the axis of the turbomachine, towards the end wall of the chamber, constitutes an improvement since it reduces the pressure drop between the outlet from the diffuser and the upstream end of the injection system, by feeding these systems more directly. It also enables the outer and inner bypass zones of the chamber to be fed with gas in more symmetrical manner, and also provides a better gas feed to the inner bypass zone. Furthermore, the feed of gas to the various admission channels of each injection system is likewise more uniform.